Selective laser sintering is a relatively new method for producing parts and other freeform solid articles in a layer-by-layer fashion. This method forms such articles by the mechanism of sintering, which refers to an process by which particulates are made to form a solid mass through the application of external energy. According to selective laser sintering, the external energy is focused and controlled by controlling the laser to sinter selected locations of a heat-fusible powder. By performing this process in layer-by-layer fashion, complex parts and freeform solid articles which cannot be fabricated easily (if at all) by subtractive methods such as machining can be quickly and accurately fabricated. Accordingly, this method is particularly beneficial in the production of prototype parts, and is particularly useful in the customized manufacture of such parts and articles in a unified manner directly from computer-aided-design (CAD) or computer-aided-manufacturing (CAM) data bases.
Selective laser sintering is performed by depositing a layer of a heat-fusible powder onto a target surface; examples of the types of powders include metal powders, polymer powders such as wax that can be subsequently used in investment casting, ceramic powders, and plastics such as ABS plastic, polyvinyl chloride (PVC), polycarbonate and other polymers. Portions of the layer of powder corresponding to a cross-sectional layer of the part to be produced are exposed to a focused and directionally controlled energy beam, such as generated by a laser having its direction controlled by mirrors, under the control of a computer. The portions of the powder exposed to the laser energy are sintered into a solid mass in the manner described hereinabove. After the selected portions of the layer have been so sintered or bonded, another layer of powder is placed over the layer previously selectively sintered, and the energy beam is directed to sinter portions of the new layer according to the next cross-sectional layer of the part to be produced. The sintering of each layer not only forms a solid mass within the layer, but also sinters each layer to previously sintered powder underlying the newly sintered portion. In this manner, the selective laser sintering method builds a part in layer-wise fashion, with flexibility, accuracy, and speed of fabrication superior to conventional machining methods.
The selective laser sintering process, and apparatus for performing the process, is described in further detail in U.S. Pat. No. 4,863,538, issued Sep. 5, 1989, U.S. Pat. No. 4,938,816, issued Jul. 3, 1990, U.S. Pat. No. 4,944,817, issued Jul. 31, 1990, and PCT Publication WO 88/02677, published Apr. 21, 1988, all of which are incorporated herein by this reference.
While the selective laser sintering process has been successful in generating accurate and three-dimensionally complex parts, certain unfavorable attributes of the process and resultant parts have been observed. Firstly, the scanning described in the above-referenced patents and publication has produced parts which are structurally weaker in one direction than another. Secondly, it has been observed that the resolution at which the parts are produced have a directional dependence, particularly at those edges of the part which are nearly parallel with the direction of scan of the laser. This is because the location at which the laser can be started and stopped (i.e., the ends of laser scan line) can be controlled to within a 0.001 inch tolerance, while the width of the laser spot size is somewhat larger, for example on the order of 0.020 inches. Thirdly, thermal warpage, or curling, has also been observed to be directionally dependent.
It is therefore an object of the invention to provide a method of selective laser sintering which can produce parts which are isotropic in construction.
It is a further object of this invention to provide such a method which has improved resolution in a manner which is directionally independent.
It is a further object of this invention to provide such a method which reduces warpage of the part being produced.
Other objects and advantages will be apparent to those of ordinary skill in the art having reference to the following specification, together with the drawings.